Journal of Geosciences-osaka City University Vol. 16， Art. 5， P. 69-91. March， 1973

F~cal， MechaJlism of Small Earthq~akes in the Kii

Peninsu.la， Kii Channel and' Shi'koku" Southwest Japan

and S'ome. Problems R'elatedω ぬePlate Tectonics*

Kiyoji SmONO

(with 1 Table and 11). Figures)

Introduction Data

Contents

Method to determine focal mechanism solutions Mechanism sollutions for very shallow earthquakes Tectonic impIication of high seismic activity in the Wakayama region Mechanism solutions for relatively deep earthquakes in the Kii peninsula， Kii channel and Shikoku Collision of the PhiIippine Sea plate against the Asia plate Supplementary remarks Conclusion and Acknowledgement Appendix

Introduction

According to the hypothesis of the “Plate Tectonics"， Southwest Japan is located on the western oontinental side of a zone of plate convergence between the Asia and the Pacific

plates and also in the nortern side of a zone of convergence between the Asia and the Philip-

pine Sea plates (Fig. 1). Some models have been proposed， on the basis of ，crustal move-ment after the Nankaido earthquake (FITCH & SHOLTZ， 1971)， the radiation patterns of P-waves first motions from great earthquakes such as the 1923 Kwanto， the 1944 Tonankai and the 1946 Nankaido earthquakes (KANAMORI， 1972b) and topographic and geologic evidence (ANDO， 1972; SUGIMURA， 1972). However， the spatial extent of these models is global， so that it is difficult to correlate them directly to tectonic movement in Southwest Japan in the Recent stage. In order to discuss geologic evidence of crustaI movement in

terms of the hypothesis of the “Plate Tectonics"， it will be necessary to understand the ex-pected geologic and geophysical appearances of the coIlision of the oceanic lithosphere against

the continental one. Focal mechanisms of small earthquakes might also provide seismo-

logical evidence to this problem， as studies on focal mechanisms using data from the world wide station net have given various useful informations on the global tectonics.

The main purposes of this paper are to test whether seismicity and focal mechanism of

smaI1 earthquakes in the Kii peninsula， Kii channel and Shikoku can be interpreted in terms

* Contribution from the Department of Geosciences， No. 264.

Foca! Mechanism 01 Small Earthquakes in Kinki and Shikoku

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Fig. 2. Seismic activity in Southwest Japan. Epiωnters (upper) are taken from the Regional Catalogue of Earthquakes in and near Japan (1961-1970) published by the J.M.A. and

vertical sections in southern Chubu (Iower right) and in the Kii peninsula (lower Ieft)

are redrawn from YAMADA & OOIDA (1972) and from KANAMOR1 & TSUMURA (1971).

It seems that relatively deep shocks (H> 30 km) form a inclined seismic belt of a smaIl scale. I，t is also to be mentioned that no shocks are deeper than 100 km .

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M.A.， respectively. Mechanism solutions are given for 61 smaJ1 shocks located in the area surrounded with a solid line.

、

72 Kiyoji SmONO

channel and Shikoku， 61 earthquakes are selected from the Monthly Seismological Bul1etin

of the J.M.A. for the period from 1967 to 1969. Their mechanism solu，tI:oss are determined through the procedure described i's the fol1t)w4ng section.

Method to determine focal mechanism solutions

The Iocation， depth and origin time of the earthquake， and emergent angle at the focus to each station are determined from the least squares by the use of a computer program

similar to that developed by AKl (1963a， b)， which is appropriate to a medi:um which consists

of two sperical shells. It is assumed that P-wave velocity varies as v=vo (r/ro)Zo in the upper

shell corresponding to the crust and as v=v1(r/rly~1 in the upper mantle， where r i is the

radius of the outer surface of the shell from the earth's cenrter and V i is the velocity at r i・ Inthe present computation， we use the foHowing parameters，

vo=5.78 km/sec

Zo= -24.4

vo=7.75 km/sec zo=-2.3

(CJBP1 in A幻 (1963b))

ro=6355.54 km

(radius of curvature of the meridian at 340N)

1"1=6318.54 km

because it has been demonstrated by AKI (1963b) that the calculated travel times from this model Sh0W a reasonable agreement with the observed travel times from tl1e Miboro 'ex-

plosion carried out in Central Japan by the Research Group for Explosion Seismology (R.

G.E.S.， 1961). The arrival times of P-waves recorded at the microearthquake observation stations and the seismological stations of the J.M.A. are used for the location of hypocenter， where the weight of observed data from the latter stations is taken to be 1/3 of that from the former stations. The coordinates of earthquake focus and origin time determined by the

J.M.A. are input as first approximation. If the traveI time residual (0・C)at a station exceeds 2 seconds on a step of least square computations， the station is excluded here-after. After less than 5 repeated calculations， the final soiution cOlilverges into a re圃

asonable value.

BecauS'e thお oomputatiomgives siDlJtdtameoMsly the' emergemt angle of P圃 waveto each

station， it is easy to project the initial motiions of P圃 waveson the upper hemisphere of the

Wulf-g伊Ird.ti

Mechanism solutions determined through the above procedure are shown in Appendix.

Open and closed circles are dilatation(pull) and compression (push)， and P and T give the axes of maximum compression (the P axes) and minimum compression (the T axes)， respec輔

tively. Origin time， coordinates of focus and the trend and plunge of the P and T axes are

tabulated in the annexed Table. 19 solutions with an asterisk (勺 havebeen determined in a previous paper (SHIONO， 1970)， while 3 solutions with double asterisks (**) are newly determined here from the radiation patterns given in the paper (See Appendix). It should

be mentiomed that the ooordinates of foc't1ls" origin times and ，emergent angles in the case of earthquakes I!n the northwestern Kii peninsula (the Wakayama region) were calculated in the previous paper on a computer program developed by MIKUMO et al (1970) assuming a

more realistic crustal structure appropriate to a limited area. We assume the simpIified

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Epicenters of small shocks (January 1965-June 1965) in the Kii peninsula. Redrawn from KANAMORI & TSUMURA (1971); (a) sha110wer than 10 km and (b) deeper than 10 km (1. 10<Hζ30 km， 2. 30<H <40 km， 3. 40くHζ50km， 4. 50<H <60 km， 5. 60<H <70 km).

Note that no shocks occurred at depth below 70 km.

Fig.4.

、

74 Kiyoji SmONO

structure mentioned above for the other earthquakes， because a precise velocity structure

over the Outer Zone (Pacific side) of Southwest Japan has not yet been known well.

Mechnism solutions for very shallow earthquakes

Northwest part of the Kii peninsula (the Wakayama region) is well known to be

a area of high seisn1icity. In order to make clear the seismicity and focal mechanism of

earthquakes in this area， various seismological and geodetic investigations have been made by lMAMURA (1928， 1929)， MIKUMO (1956， 1959)， MIYAMURA (1959， 196め， MIYAMURA &

OKADA (1960)， MIYAMURA et al (1966)， WATANABE & KUROISO (1967)， MIKUMO et al (1970)， SHIONO (1970)， KANAMORI & TSUMURA (1971) and many others. The Wakayama Micro

earthquake Qbservatory established in 1964 by the Barthquake Research Institute， the University of Tokyo has made extensive observations of microearthquakes with about ten

satellite stations distributed widely in the Kii peninsula.

Fig. 4 shows the epicenters of microearthquakes with depths shollower than 10 km

(a) and deeper than 10 km (b)， (redrawn from KANAMORI & TSUMURA (1971))， which occurred in the Kii peninsula during the period from January 1965 to June 1965. It can

be seen that the north side of the active area near Wakayama (WK) and Arida (AR) is

bordered by the Median Tectonic Line which has evidence of right帽 lateralslip in the geolo-

gical1y recent times (OKADA， 1968， 1970; HUZITA & OKUDA， 1973). On the other hand， the west side of the area seems bounded by a certain kind of a tectonic line trending north to

south， whioh runs paraHel to the general trrend Qf the ，coast line of northwestern Kii peninsula. Aotive zone near Hidaka (HD) may be probably connected at its west esd with an active

zone of relatively deep earthquakes in the Kii peninsula (Fig. 4b)， but the detaHs of this continuation have not yet been investigated sufficiently.

In the present paper， 32 mechanism solutions are given for very shallow earthquakes in the northwest part of the Kii peninsula for years 1967-1969. Magnnitudes of these

shocks range from 3.1 to 5.0.

Fig. 5 shows the epicenters and mechanism solutions projected on the upper hemisphere

for 25 shocks near WK and AR. Open and shaded parts in the mechanism solutions

denote compressional (push) and dilatational (pull) areas， respectively. A smal1 open circle in each ofsolutions gives the P axis. Numbers for the solutions correspond to the identification

numbers in the (l)ooexed Table. From tbe above figure， it appeaFS that there is a variety of

solutions， but we can recognize two interesting features. One is the feature that the P axes are nearly horizontal1y striking eastward， whereas the

T axes are distributed at random on a plane perpendicular to the average direction of the

P axes (Fig. 6). The other is that motion on the fault plane of almost all solutions have large components

of thrusting. 9 solutions shows pure thrust faulting (NOs. 5， 12， 13， 14， 15， 17， 18， 21 and 25). Both evidence seem to suggest consistent1y that these very shallow earthquakes are

generated by horizontal compression oriented in the B-W direction.

Si'milar features can be seen in mechanism solutions of shocks near Hidaka (HD)

(Fig. 8'). A】tboughthe active Zlone of very ぬallowearthquak!es in the northwest Kii pe-

ninsula seems to continue near the off-shore paFt of Hidaka to an active zone of relatively

deep earthquakes inside the Kii peninsula， we do not obtain a large number of mechanism

solutions enough to discuss the relation between two active zones， because of lower activity

Focal Mechanism 01 Small Earthquakes in Kinki and Shikoku 75

21

22

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1350 15'

Fig. 5. (left) Mechanism solutions of very shal10w shocks (NOs. 1-25) in a seismically active area near Wakayama (WK) and Arida (AR). Open and shaded parts in solutions are di1atation (ptdl) and eompression (push)， respectively， smaU open cirtc1e in each of solutions gives the P axis and numbers correspond to the identification numbers in the annexed Table.

Fig. 6. (right) The P (open circles) and T (closed circles) axes of 25 shocks shown in Fig. 5. They are projected compositely on a upper hemisphere of the Wulf-grid.

as compared with that of Wakayama-Arida area.

A small earthquake-swarm occurred in 1969 near ChihayルAkasaka，east part of Osaka prefecture (See Fig. 2 and 8). Shocks NOs. 44 and 45 are the two greatest earthquakes

included in the sequence of this swarm. The P axes are also nearly horizontally striking

eastward. It is to be mentioned that a thrust fault (the Kongo fault) runs north to south， 5

km east of epicenters. Since mechanism solutions of the two shocks can be interpreted as

strike-slip faulting (See Appendix)， it may be inferred that this earthquake-swarm is not

due to thrusting along the Kongo fault.

Mechanism solutions for very shallow earthquakes Iocated in eastern Shikoku (NOs.

47-51) also yield the P axes oriented nearly horizontally in the E-W direction (Fig. 8). SA-

WAMURA & KIMURA (1971) showed that very shallow earthquakes in central Shikoku were

generated by the E圃 W compression. Unfortunately， as the details of seismicity in eastern Shikoku has not been obtained， we cannot discuss satisfactorily a possible relation between

seismicity and focal mechanism.

As described above， the P axes of very shallow earthquakes in both western Kii penin-

sula and Shikoku generally strike eastward. These features are similar to those of very

shal10w earthquakes in northern Kinki and Chubu regions (NISHIDA， 1973; OOIDA & ITo， 1972). Fig.7a shows horizontal components of the P axes of very shollow shocks in Sou-

thwest Japan.

、

76 Kiyoji SmONO

Tectonic implication of high seismic activity in the Wakayama region

Seismic activi1ty of very shallow earthquakes in the northwest Kii peninsula (the Wakayama region) falls off not only in the north side of the Median Tectonic Line but also

in the west side of a line parallel to the general trend of the coast line， which can be interpreted

to reflect the Recent tectonic movement. HUZITA et al. (1973) proposed the existence of a

thrust fau~t (the Kii thrust) along the west border of the area of high seismicity striking

360

1340

-360

1340

G

H{30 km

b H)30km

、a、、

¥380

、、• •

1380

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Fig.7. Horizontal components of the P axes of (a) very shallow shocks (Hζ30 km) and (b) relatively deep shocks (H> 30 km). The P axes of shocks in northem Kinki and in eastern Kinki and Chubu are taken from NISHlDA (1973) and OOIDA & ITo (1972)， respectively. NISHIDA (1973) gave 53 solutions of small shocks (Mz2.5) for years

1966-1969 and OOIDA & ITo (1972) gave 138 solutions of small shocks (Mミ2.2)for years 1968-1971. Some of the P a克eωs0ぱfverηy shallow shocks (H三ζ三30km) in the

Ki泊nkia釘reωaaMre Ormm|Ennrnnll4i加iwho wish to know precise distribution of the axes should refer SmoNo (1970) and NI路SH郎王葺IDA(ο19卯73め). In Fig. (b)， horizontal component of the axis with plunge greater than 45 0 is 110t shown (closed circle).

Foca/ Mechanism 01 Small Earthquakes in Kinki and Shikoku 77

nearly is the N-S direction， on the basis of the foIJowing evidence; seismic activity is closely related with active faults in other regions of Southwest Japan (for example，the Yamasaki fault and the Neodani fault， see HUZITA & KISHIMOTO， 1972; WATANABE & NAKAMURA ..

1960; OOIDA et a!， 1971; MATSUMURA & OIKE， 1973) and this portion of the Kii peninsula corresponds to the flank of foundation folding where a thrust fault tends to be generated in a final stage of folding activity.

As described ia the preceeding section， the P axes are roughly horizontal and oriented

in the E-W direction， and there are many earthquakes that have the mechanism with large components of thrusting. Although no destructive earthquake along this line has been do-

cumented in the historical records， above-mentioned features suggest strongly the existence of the thrust fault. If the existence of this thrust fault is accepted， it is possible to inter-prete the high seismic activity in the Wakayama region as due to a local high stress concen-

trated near a corner of the crustal block bordered by the Median Tectonic Line and the presumed Kii thrust.

This fault may probably terminate near the off-shore part of the Hidaka area， because the seismic activity falls off south of Hidaka， and because the Butsuzo tectonic line divides the west Kii peninsula into the northern block and the southern block， near Hidaka， in the simHar manner as tlhe Median Tectonic Line devides SouもhwestJapan into the Inner Zone

(Japan Sea side) and the Outer Zone (Pacific side). It is to be mentioned， however， that move-ment of the Butsuzo tectonic line in the Recent stage has not yet been studied enough， unlike the Median Tectonic Line.

Mechanism solutions for relatively deep earthquakes

in the Kii pe凶nsula，Kii channel and Shikoku

Relatively deep earthquakes (H>30 km) occur in a narrow belt along the Outer Zone

of Southwest Japan (Fig. 2). These earthquakes form a inclined seismic belt of a small

scale， although it is only poorly developed (Fig. 2; see KANAMORI & TSUMURA， 1971; OOIDA & YAMADA， 1972; YAMADA & OOIDA， 1972; SAWAMURA & KIMURA， 1971; KANA-MORI， 1972b). It has been pointed out that these mechanisms are quite different from the

case of very shal10w shocks in the same region. The P axes of these shocks in southern

Chubu (OOIDA & ITo， 1972)， Kii channel (SHIONO， 1970) and central Shikoku (SAWAMURA & KlMURA， 1971) are roughly oriented in the N・Sdirection. Therefor， activity of these shocks might be due to a regional stress different from a general tectonic stress working in

the crust of Southwest Japan. On the other hand， many authors proposed underthrusting ofthe Philippine Sea plate along the Nankai trough beneath the Asia plate， on the basis of seismological evidence (KANAMORI， 1972b; FITCH， 1972)， geodetic evidence (FITCH & SHOLTZ， 1971) and geologic evidence (Ando， 1972; SUGIMURA， 1972). The present 29 solutions and

solutions determined by OOIDA & ITo (1972) are compiled together in Fig. 5b， which seems to persist convincingly that activity of these relatively deep shocks are due to a regional tec-

tonic stress related to the underthrusting of the PhiIippine Sea plate.

Precise studies on seimicity and focal mechanism of relatively deep earthquakes will

provide further evidence on the manner of interaction between' the Asia and the Philippine

Sea plates. From this point of view， let us examine 29 mechanism solutions for these sho国

cks in the Kii peninsula， particularly， Kii channel and Shikoku (Fig. 8). The mechanism of

small deep shocks beneath the west Kii peninsula may be classifi.ed into three types. 3 shocks

，

30・32

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Fig.8. Mechanism solutions for small shocks in the Kii peninsula， Kii channel and Shikoku. Epicenters are given by open (H <30 km) and closed (H>30 km) circles， and the P (open circles)， T (closed circles) and null (cross marks) axes of shocks with sinular mechanism are projected compositely on the upper hem，isphere of the Wulf-grid. Mechanism solutions of 3 shocks (NOs. 44 46) and 25 shocks (NOs. 1-25) are not shown in thisfigure. For refereuce， contour lines of a plane striking parallel to the Nankai trough and dipping northwest-ward gently (dip of 200

) from the inner wal1 of the trough are rough1y shown by fine Iines and depth of hypocenters are given by numbers near epicenters.

Foca! Mechanism 01 Small Earthquakes in Kinki and Shikoku 79

located in the Kii peninsula (NOs. 41-43) are the deepest ones (Hキ 70km) and have mccha-

nism solutions indicating strike-slip motion with the P axis striking northwestward， ¥tvhich is nearly parallel to the direction of underthrusting of the Philippine Sea plate (KANAMORJ， 1972b; FITCH， 1972). This type of mechanism is termed here as the A-type. 3 shocks

located near the coωt Hne (NOs. 38-40) have fl1echanisn1 solutions indicating nOflnal-s1ip

motion. The P axes dip steeply. Hypocenters are shaIlower than shocks of NOs. 41-43

(日キ50-60km). This type of n1echanism is termed here as the B-type. 5 shocks located in

Kii channel (NOs. 33-37) have lnechanism soIutions indicating strike-slip motion ¥tvith nearly

horizontal P axes striking northward. Hypocenters are slightly shalIower than those of

NOs. 38-40. This type of mechanisl)1 is termed here as the C圃 type.

Mechanism solutions for relatively deep earthquakes in Shikoku can be also classified

into the B-or C-types. 5 solutions (NOs. 54-56， 57-58) are of the C-type， whereas 4 so・

lutions (NOs. 52-53， 59， 60) are of the B-type， although shocks of NOs. 59 and 60 nlay be classified in to the C岨 typebecause of Iarge amhiguity. There is no shock with the A-type

mechan.ism. If we would choioe more realistic crustal structure and determine locations of

hypocenters and emergent angles of P-waves at foci， the locations and the radiation patterns

of the initial motions on the focaI spheres rnight sonlewhat differ from the present ones. Ho-

wever， they might not vary critically because the travel time residuals in the present computa-tion are reasonably small (See the annexed Table) and there are SOlne stations near epicen-

trers (See Fiι3) except for shocks of NOs. 57四 61，whose solutions are less reliable because of lack of seisn1010gical stations near the epicenters. Therefore， above-mentioned classiflca-tion can be regarded to be reasonable in high probability.

Considering mechanism solutions determined by OOIDA & ITo (1972)， sI1ni1ar classifi-cation also seems to be applicable for relatively deep shocks in southern Chubu.

It is interesting to infer the stress state in the upper 1Tnant1e of the Outer Zone of

Southwest J apan from 3 types luechanism. One possible inference will be discussed in the

next secI1OrI1.

Collision of the Philippine Sea plate against the Asia plate

KANAMORI & TSUMURA (1971) and KANAMORI (1972， b) stated that NE-SW trending

wedge-like distribution of earthquakes with depths ranging from 30 to 70 km was relnarkable

along a line connecting two stations ST and KK in the Kii peninsula (Fig. 4b)， and that this activity represented the results of col1ision of the Philippine Sea plate against the Asia

plate. Their interpretation for clustering of slnall earthquakes at depths around 50 km and

the complete absence of earthquakes at depths d~eper than 80 km is that a leading edge of

the underthrusting plate migbt be reach to a depth of only 50 to 70 km and a distance of

50 km inland. This interpretation may be applied to the zonal activity of relatively deep

earthquakes in the Outer Zone of Southwest Japan， which inclines gently northwestward.

If these earthquakes represent some activities in the continental Iithosphere， a possible

inference may be drawn;

Let us imagine deformation of the margine of the Asia plate， beneath which the Philip-pine Sea plate underthrusts. Suppose the case that the rigid oceanic plate， of which leading edge remains in the continental plate， gives a constant rate of displacement to the interface

between the two plates，. The interface is assumed to be locked tightly and to offset only

at the occu汀 enceof great earthquakes such as the 1944 Tonankai and the 1946 Nankaido

• ヘ--

W8

C

80 Kiyoji SmONO

A

〉矢

沢

• .・Q o

• 3えフ氏、，"1-、， )く

)(')( • 。

M T I.i NT

010

'々Fig.9. Upper; three typ~s mechanisms. The A， B and C-types include 3 shocks (NOs. 41~

43)， 7 shocks (NOs. 38-40， 52--531 8l1i1d 59-60) amd 10 shecks (NOs. 33-37 and 54-58)， respectively. Symbols are as same as those of Fig. 8. Lower; schematical diagram showing tentativelty the deformatiol! of the marginal part of the Asia pla:te (vertical section). Fine lines show emphatical1y the deformation of the initialIy vertical planes due to undertrusting of the Philippine Sea plate and thick bars with central closed circles denote axes of maximum compression. MTL; the Median Tectonic Line， NT; the N ankai trough.

earthquakes. The displacement wil1 cause deformation in the continental plate， as shown in Fig. 9. In a oompressed area near the leading edge of the ooeanic plate， the axis of maximum compression will be near]y paral1el to the direction of underthrusting and dipping

gently northwestward. Earthquakes located in this area may have the A-type mechanism

termed in the previous section. In a dragged area just above the interface， the axis of maximum compression will dip steeply. Earthquakes located in this area may have the

B-type mechanism. In a area somewhat distant from the interface， the axis of maximum compression will be nearly horizontal or dipping gently. In order to demonstrate that

earthquakes in thiS' area shouid have the C-type meehanism， it is necessary to 'explain why the axis of maximum compression is not parallel to the direction of underthrusting of the

oceanic plate but oriented in the N-S direction. This should be s'Olved in the near future， but it may be partly because oblique underthrusting (KANAMORI， 1972b; FITCH， 1972) gives horizontal shear as well as compressional strain to the continental plate and partly because

the E圃 W comtJ)onent of displacement in the continental plate translates westwards the

Outer Zone of Southwest Japan probably south of the active Median Tectonic Line so that

theE圃W component of stress is releasea in part and the N-S component remains (See Fig. 10).

. According to this interpretation， it is inferred that the hypocenters of shocks with the

;、 /; 凡ーーーー--~

81 Foca! Mechanism 01 Small Earthquakes in Kinki and Shikoku

b q at__ 1 1 t J • t 1 1 I I

Jm，，，パV

，，J

CJLφl

↑afff

Schematical diagram showing the effect of oblique underthrusting and right-lateral slip along the Median Tectonic Line. Oblique undeFthrusting of the Philippine Sea plate gives horizontal shear as well as compression (b) and slip along the Median Tectonic Line releases the E-W compression in part the and N -S compression re-mains (c).

Fig. 10.

A-type mechanislll wiI1 be deepest and those of shocks with the C-type mechanism will be

generally shallower than those of shocks with the B-type mechanism. These inference are

nearly consistent with' the observations described in the previous section.

Su，pplemcntary remarks

The fact that very shal10w earthquakes in the northwest part of the Kii peninsula (the

Wakayama region) have the mechanism solutions with nearly horizontal P axes (axes of

maximum compression)， striking roughly eastward， supports the idea that these earthquakes are also the consequence of a general tectonic stress in the margine of the Asia plate caused

by u1llderthrusting of the Pacilfic plate， in the same manner as very shallow shocks in northern Kinki and Chubu area. However seismic activity in this area seems to correlate with the

occurrence of great earthquakes along the Nankai trough such as the 1944 Tonankai and

the 1946 Nankaido earthquakes (KANAMORI， 1972a， for example). The average direction of

the P axes (N 1050E; SHIONO， 1970) differs slightly from that of very shallow shocks in northern Kinki area (E-W; NISHIDA， 1973). These evidence suggest that underthrusting

of the Philippine Sea plate may play a role to seismic activity in this area (KANAMORI， 1972a)， but because the P axes strike roughly eastward， the role lnay be a secondary one.

Recently， MIZOUE & NAKAMURA (personal comunication) pointed out the linear align-

ment of epicenters of microearthquakes in this area forms a small scale grid輔 netof about

several ki10meters interval and suggested that this area may be divided into many small crustal blocks. It is interesting to infer that stress concentration as described in the

previous section has broken down this area into small pieces. However， it is necessary

to test whether precise study of crustal movement in the Recent stage may support above

inference. This is an important problem in order to understand the tectonic process in this

area and is to be solved in the near future.

On the other hand， it is inferred that relatively deep earthquakes in the Outer Zone

of Southwest Japan have resulted from the collision of the Philippine Sea plate against the

Asia plate. However， mechanism solutions of these earthquakes do not exactly reflect the di圃

rection of the underthrusting， unlike those of deep and intermediate earthquakes in the well

、

82 Kiyoji SmONO

developed descending slab of lithosphere (for example， ISAcs & MOLNER， 1971; OIKE， 1971). The cause of this dissimilarity may be part1y becalI1se magnitudes of the relatively deep earth-

quakes discussed are considerably smaller than those of the earthquakes discussed in the

globa1 tectonics， and part1y because the present earthquakes represent seismic activity in the continenta1 p1ate deformed by underthrusting ofthe oceanic plate. ICHIKAWA (1971) stated

on the basis of data from the J.M.A. stations that mechanism solutions of shallow shocks

in Southwest Japan， particularly near Kyushu， may not be consistent with the hypothesis ofthe “P1ate Tectonics"， but it is necessary to re固examineprecisely his soJutio1ils considering

above-mentioned remarks.

Conclusions and Acknowledgements

Using data from moωre than 3'0 s鈎eismologi比cais幻tatiぬIoOInmEm S wt仇tbhi

6“1 mechanism solutions have been obtained for smal1 earthquakes with magnitudes down

tωo 3幻10ωca拭te“di泊nthe Kii peninsula， Kii channel and Shikoku， Southwest Japan. Very shallow

earthquakes in the northwest part of the Kii peninsula (the Wakayama region) and eastern

Shikoku have mechanism solutions with nearly horizontal P axes (axes of maximum

compression) striking roughly eastward. lt is concl'uded tbat these earthqruakes in the

Outer Zone of Southwest Japan are also due to a general tectonic stress caused by under-

thrusting of the Pacific plate beneath the Asia plate， similar to the case of very shallow earthquakes in the Inner Zone. Mechanisro solutions have large components of thrusting

for very shal10w earthquakes in the area of high seismic activity (the Wakayama region) and

seismic activity decreases abruptly not only in the north side rof the Median Tectonic Line

but also in the west side of a line paralle1 to the general trend of the coastline， east side of Kii channe1. HUZITA et al. (1973) proposed the existence of a thrust fault along this line (the Kii

thrust). Mechanism also favors this proposa1. Therefor high seismic activity in this area

may be interpreted as due to stress concentration near the corner of the crustal block.

Re1atively deep earillquakes (H> 30 km) in the Kii penin側 [a，Kii chaUllel and Sb比oku

have quite different mechanisms from those of very shal10w earthquakes in Southwest Japan.

Mechanism solutions support the hypothesis proposed by KANAMORI (1972b) that these

earthquakes in the Kii peninsula represent the collision of the underthrusting Philippine

Sea plate against the Asia plate. Close examinations reveal out that these mechanism

solutilons can be c~ass泊ed into three types. The A-type mechanism has the horizontal P

axis nearly parallel to the direction of underthrusting of the Phili_ppine Sea plate. The

B-type has the P axis dipping steeply. The C-type has the P axis oriented in the N司 Sdirection

near1y horizontally. If these relatively deep earthquakes represent the seismic activity in

the continental plate， it is possible to infer the stress state in the portion of the Asia plate near the interface between. the oceanic and continemtal ptates. From this point of view， a

possible inference has been tentatively given to explain three types of mechanism.

As a result， these mechanism solutions of small earthquakes not only can be interpreted in terms of the“Plate Tectonics" but also seem to give a clue to make clear the interaction

between two plates， the Asia and Philippine Sea plates， so that precise examinations need to be performed in other area of Southwest Jap釦 fromabove-mentioned viewpoint.

1 wish to thank， for providing data of initial motions， the Wakayama Microearthquake Observatory of the Earthquake Research Institute， the University of Tokyo， the Tottori Microearthquake Observatory of the :qisaster Prevention Reserch Institute， the University

Focal Mechanism 01 Small Earthquakes in Kinki and Shikoku 83

of Kyoto， the Abuyama Seismological Observatory， the University of Kyoto， the Kochi Seismological Observatory， the University of Kochi， the Inuyama Microearthquake Obser-vatory， the University of Nagoya and the Japan Meteorological Agency. 1 would like to

thank DR. T. MIKUMO and PROF. Y. KISIDMOTO for helpfull suggestions and a critical

reading of the manuscript. 1 also would like to express my appreciation to DR. K. OIKE

for frequent stimulating discussions and to PROF. K. HUZITA for usefull suggestions from

a geologic point of view.

Appendix

As is mentioned in the previous section， the source parameters and mechanism solutions are summarized in the annexed Table and Fig. 11-(a)， 11-(b)， 11-(c) and 11-(d). For the

expressions， one should refer to the paragraph in p. 72 .

•

∞ 。』

，

Source parameters of smal1 shocks in the Kii peninsula， Kii channel and Shikoku， South-west Japan for years 1967-1969. Origin times are given in terms of the Japan Mean Time (J.M.T.)， a is the square route mean of the residual time (O...C)， trends (Tr) and plunges (Pl) of the P and T axes are measured clockwisely from the north and upward from horizontal planes， respectively ， R gives rank of arpbiguity in determining fault planes (a; less than土5。，b;less than土100，，c; greater than土10'0)，M is magni伽 d@detern:nl!ined by the J.M.A.， soh.ltions with an asterisk(*) have been determined in a previous paper (SmoNo， 19'7(1))， solmtions with double asterisks(林)are newly determined here from the radiation patterns given in the paper and numbers in column Ref. correspond to the solution num-bers in the paper.

Table.

Remarks

M Ref. R

axlS

Pl.

T

Tr.

関山句&即日山田。

zo

S・10

S・12S・13S-14 S・16

15392

4343司

343a匂

aaaba

400

'40

14

38

61

40

43 355 15 0

P axis H Tr. PL

5Kln 2650 120

5 313 12

4 90 14 4 280 8

4 117 13

-mL

・時

bo

Long. 。Origin Time

h m S

Date

(J.M.T.)

Solution

No.

5.5'

3.9

11 .4 10.8

9.7

340

34

34 34

34

9.4 7.7

14.2

9.8

12.2

1350

135

135

135

135

0.063

0.100 0.086

0.030

0.194

21.8 11.2 31.9

47.7 40.。

弓

I司

3弓

I'i唱

i

司

3tA司

3司

3

a品

T

'i弓

I弓

I司，，M

』品T

-A唱

A'i今，BAU

14，1967 27，1967 11， 1967 22， 1967 29， 1967

Apr.

May Jul.

Sep.

Sep. *

*

*

*

*

*

曹

A司，b

司

3

4且寸

ξJ

S-17 S・18

S・19

S・23

3.6

3.1

3.7

3.7

accab

84226

34241

47 30

215 30

27

1Y6 8

45

0 11

306

293

125

105

292

』

ιT品品

T

弓，a且T'A

-EA

!LiQ.5 10.4

18.1

8.1 5.6

a品T泊

噌

』

品

T

A守

A守

qd2dqd司

32d

12.9

12.5 17.8

9.4 4.7

135 135

135 135

135

01.@64

0.053 0.042 0.036

0.359

3.@

59.2

59.5

31.2

17.5

432dnuζuqJ

tiA守泊

匂

AU43

ootifO司

I今，H

AU仇UAUti'i

29， 1967 30， 1967 2， 1967 5， 1968

19， 1968

ppvbL

eeoea

S

S

N

F

M

6* 7* 8*

9*

10

S・27

S・28

S・31S-32

S-35

5.0

3.9 3.6

3.3 4.0

aaaaa

』品

ToorOAU司

I

'irooo'AOO

27

175

80 333 272

司

J4JH』品

TCJq3

294

276 260

89 92

aaT』品

TAY』品

Tfo

11.2 12.3

8.2 8.0

11.8

a且T』品

τ

8品τ

8且

T

A品T

qd-qdミd弓

32d

11.7 11.7 5.0

10.2

7.6

135

135 135

135

135

0.080

0.184

0.118 0.037

0.085

32.1

40.5 28.1 42.2

20.6

A品τ

O

Y

2

0

。ベ

M

AunU』品寸今

L-A

MW000ω

30， 1968

30， 1968 22， 1968

6， 1968 8， 1968

Mar. Mar. Apr.

May Jun.

11* 12* 13** 14本*

15**

S-41 S-48

2504・A

司

3司

3

a且TauTa斗

a b

aaa

nuoonUAUnu

a斗

fO今''M

8守

43

22 260

27 175

9

今

'unO司、

dAUζJ

司

3'i'iq343

262

105 266 285

123

a且Tro--司

J

fo

-EA

5.4 10.0 4.6

2.4 11.6

a品TaιTSιT泊品τ』品τ

qdqdqd2J43

12.4 12.0

7.7 7.1 7.2

135 135

135 135 135

0.044

0.151 0.431 0.054 0.102

39.4 55.3

11.6 40.2 50.9

ハU勺

IAUA守

2i

'iζJ司

3

d守

司

，b

司

Junonunyny

nunU

官

i'i

12， 1968 6， 1969

15， 1969 31，1969 9，1969

川市均

M

W止

AFUMMh

16* 17*

18 19* 20*

S-57 S・59

3.5 3.4 4.5

3.5 3.6

bbabb

fony'inucJ

ZJauマ

ξJ唱

i

ro

15 330 38

209 40

fo司

I守

，

司

3

ro

司、d

唱

i司

3

273

120 285 292 300

勾

f弓

I今，U43AU

‘.A

14.1 12.7 4.3 4.9

13.7

A品1A品Y

A守』

品

TS且T

43vq3司

343司

3

9.8 7.5 4.3 4.2 11.3

135 135 135 135 135

0.120 0.099 0.420 0.244 0.608

26.8 40.7 21.9 19.3 38.0

a品Tnuoo'iq3

nu--A守

ζJ

phJ

ORuq3冒

i

AU司

L

AMW唱

iAU曾

i弓

L

25， 1969 13， 1969 18， 1969 2， 1969

25， 1969

Aug. Sep. Oct. Nov.

Nov.

21* 22* 23 24 25

Remarks

Ref.

P axis T axis

Tr. Pl.

Origin

Lat. M R

、。ミミnnbむさき応、川町選。む阿佐ミ勘定ねな句刊誌尽きおミhS凡なを

S・20

S-24 S-36

i3824.

泊守a斗弓38UT』品T

b b a b

今，u』品τ8品τ

官

i

勾

''u

今J創作，&刈抽Y令

''H

7

25

32

341

1

'L

一号3rOAUζJ冒

i

D且一作，，耐えJ司，，b

今3

Tr. H

98

293

295

94

106

守，』品Tn『ZJfo

'A司，，B

54.0

57.6

57.2

52.0

51.0

司343弓3弓3弓3

弓3司343司343

7.9

20.3 20.0

18.3

1.5

Long ..

135

135

135

135

135

。Origin Time

h m S

Date

(J.M.T.)

Solution

No.

0.034

0.031

0.096

0.514

0.422

30.2

0.6

2.0

55.6

0.5

A守勺tfo

o

o

n

U

44222

弓3nu--今，B今，H

AU今，M今，b

司，BAU

15， 1967 24， 1968 8， 1968

21， 1969 25， 1961

仰

ぬ

肌

N

F

h

Mar. Aug.

26* 27*

28*

29

30

4.3

4.8

4.5

3.6

3.2

a

babac

HhJAU』且TnU勺，

A守刈守'i今，B

183

235

280

60

80

AYAU』守口RUAU

唱A

d

守

'

A

今

，H

72

100 13

327

347

fo-qunyrocJ

今，u今，BA守』品1a斗

51.6 52.1 44.8

38.1

42.3

43司343司343

司343434343

1.7 1.9

57.2

9.3

52.0

135

135

134

135

134

0.399 0.437 0.523

0.195

0.175

10.2

0.3

30.0

58.1

51.2

fo

rozJnU弓3

2

5

2

4

司IAWJ弓343勺I

、inUAU'i'i

27，1967 18，1968 30， 1967 17，1968 12， 1968

Aug. Aug. Nov. Jan. Jun.

'i今，白司3A且TZJ

司3司3434343

3.7

3.8

4.2

cbb a c

nunUAUAUJD

ぺ，，h

』品τ今，eu

60

49

235

65

52

nyAUnunU00

44今，ezJ『I43

330

331 .55 245

312

εJoony今，U司，

a品T8ATd品TζJεd

35.3

49.2

43.3 46.0

48.0

司3司3司3司3司3

43弓3弓3司3司3

9.5

2.8

17.8

8.S 17.0

135

135

135 135

135

0.337

0.565

0.190

0.313

0.397

17.9

4.9

5.9 54.5

16.4

今，uooεJa品TA守

司，B弓3'IζJ'i

cJ司I8斗000O

AU'i'A唱

i

A

U

28， 1969 9， 1969 13， 1967 9，1968 15， 1968

b.L礼

ザ

孔

-fuaιhu

rryJvJ

‘mr

J

ro勺IoonynU

434343弓3』今 4.0

4.0

3.5

4.0

3.9

aaaab

泊晶τAUAHUλ守Au

g----aA

内.，，a'EA

209

209

211

189

15

nUAUAUd品VAU

今''M

今

，

b

唱

i

114

118

113 284

105

2

9

0

6

1

ntr。『tti--

59.0

0.3

0.4 27.7

27.4

438品τ必守・8品Td品T

43司343司3司3

23.8 26.1

26.0

37.9

37.2

135

135

135

135

135

0.357

0.137

0.195

0.301 0.242

31.4 0.9

48.9

56.0

55.0

nucJ刈岨，nyny

daT43nU必斗必品7

'inucJAY』品T

AU今L官inu'A

3， 1967 12， 1968 28， 1969 18， 1969 18， 1969

Jun. Jul. Feb. Feb. Feb.

12345

d斗A伶

A『λ『必且守

4.1

4.4 3.3 4.1

3.7

bbcab

cJAU』品YZJ勺t

今JM』守勾tA品T

356 197 65

27 346

0000fofo'A

'

A

d

品Y

91 295

245

292 82

nUAUF、M守，白。

32222

48.9 52.4 43.4 45.5 41.2

43司3弓343弓3

司3『3司3司3司3

51.6 50.2

15.1

9.7 23.3

135

133 134 134 134

0.154 0.483

0.456

0.301 0.328

28.9 33.7 20.8

27.9 4.0

戸、JnyrO

今FMAY

』斗8品T43必品TA品τ

zJ今L今LA『AU

nUAU官AAU司，，u

3，1967 11， 1967 14， 1967 21， 1967 21， 1967

b.AW瓜

飢

恥

hMhD

fo司1

00nwJ

AU

8且TA守』品TA-TζJ

ooa品TζJ'ifO

34443

baaab

fo今ふ£J官

A

AU

'i'i司

L

329

68 85

299

117

'IAU』品Y'AAU

27512

62

175 199 31

19

10099

今&ζdf08品Tau守

5.9 55.1

50.0 0.3 2.3

必守43司3』品TS品τ

43内

3司343内

3

29.0 48.7 38.1

53.2 49.5

134

134 134

134 134

0.463

0.389 0.475 0.202 0.488

51.2 58.4 1.2 2.4

32.8

司3司I今L

fO

今JM

εJZJZJ弓3

F

、d

弓

3司

3

戸、dny『

l

nu官

AAUnunu

15，1968 19， 1967 27， 1967 14，1967 8， 1969

Oct. Jun. Jul S句 ・

Jul.

官

A今L

弓3

s品TF3

εJCJεJ，、JFεJ

3.8

4.0

5.6

4.5

4.2

4.1

acbbcc

司IAUZJ司3εJAU

A守

胃

A

今，b

299 110 282

239

77

10

司tnU噌

loonUAU

今

L

A品

τ

208

20 14

143

193

160

ζJξJnUξJA守今''M

ζJζJεdfozJ司3

59.4 33.9 33.8

27.9

16.0

28.3

司343弓3司3弓3司3

43司343司3司3司3

37.5

3.3 5.2

29.3

48.0

20.4

134 134

134 134

134

133

0.304

0.439 0.548

0.408 0.493 0.606

7.7 13.8 31.0 11.8 8.2

12.7

εJ今，HCJnU00弓3

qd今，&A守d品T8UTAU

守f

司'unu

fO

A

U

弓3

唱inu勺FMtA司L

nu

10， 1969 20， 1968 11， 1968 21， 1969 25， 1969 1， 1967

Dec. Apr. Dec. Mar. Jul. Jan.

ro

司InkunynU唱

i

555566

∞ c.n

86 Kiyoji SHIONO

5 (s・16) 10

•

13 (s・31)

• よ • /

。。

‘，

。 + ‘ 。

。。

.~ 。

o ¥ 勺

。 。 • ¥ . α:;> 0 •

α00。 • 、。、

•• • 。

4

';4 '15 1.8 (s・32) ，s・35) •

/。f 、 、、 。。.' ¥ ， 。。 1 ¥ / 。，.! /

。 P 。 .. 。 。¥ し

~+ 。 l勺。 。 。 ¥ 0 。 。• / ¥

、• • 。/ ¥。 ¥ 。

.0 ，

¥ / ~ ~ / ¥ ^ • 。.・. • 23 24

" 25 ~

イ¥ / ¥ • . ~ /6も、 ，

I ¥ 。 ¥ ¥ • そ〉、ど ... ¥

¥ 1づ..，. / 01

1)

J ・• 。

。0

0

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29， 30 31 •

^ バ/ 司......... / • .iよ•

L / ー

¥ / :./れ / ~ 。。 。

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fF 。

a. 1‘ 11 、 -"

" 、./

Q.

。/ ¥ • 。 /"-.... 00 。/ v ¥恥 O lJ-L ・'¥0 0 、.・。

• Fig.. 11-(a) Mechanism solutions which are newly solved in this paper. The expressions are explained in p. 72.

• ¥

Foca/ Mechanism 01 Small Earthquakes in !(inki and Shikoku 87

32 33 34

•

35

.~¥@

。

。

-・.ec，

OFJOも

もd

。

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ハ"¥ 、

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36

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f '"'" • ， ，

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37 ザぢõg~

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3，9・ ~("\.L-一____ 40 o 0'.

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o ~ 0 ¥ ¥ ( .. { ~\. ・・.¥ / 81 ・y

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ト.0.

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11 、I ¥ -

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41 42 43

ちず."， . れμ。

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Fig. t.l-(b) to be continued.

88

44.

万 。

。• ・・.•

5'0

0..

O G

-. •

Kiyoji SmONO

4.5 I

0./す百一・4

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札//

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53 54 55 4F 『 』 -F A 九 ----。。00

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• 0 0 /九ー~ • • • • • .o ー，ー'寸; 司' • '/~ ~ -.1 •

ρ ¥ • / ¥び / ¥_.............- ¥ ム、

• 。 / ¥ •

51

O ¥ 。

ηO Cレ

。

。 。o

o ef6>

5.2

• V-• ••

¥ •

。

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Fig. 111-(c) to be continued.

Foca/ Mechanism 01 Small Earthquakes in Kinki and Shikoku 89

56， 57 58 。o0 cP o~ /ぐ。 Cプ

• 。。

、 。.1 " L 。 o ¥ / ¥ 0..0

.1. ¥ ~ •• ...

• _/""¥ -J 〆r ¥ •

59 60: 61 ー I..... 、、

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Fig. 11-( d) to be continued.

Reference

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